1. Field of the Invention
The present invention relates to systems and methods for the spectral efficient transmission of signals, and more particularly, for combining transponder bandwidths for source and forward error correction encoding efficiency.
2. Description of the Related Art
Digital Direct Broadcast Systems (DBS), such as DIRECTV®, which is provided by the assignee of the present invention, have become very successful. However, as such systems evolve, there is an increasing demand for additional bandwidth to carry an ever-increasing set of audio, video and data services.
Guard bands represent an attractive source of bandwidth that is still available. For example, in the current DIRECTV® frequency plan, a symbol rate of 20 MHz and an excess bandwidth ratio of 0.2 are used for a transponder, and a guard band of 5.16 MHz exists between adjacent transponders of the same polarization. This is a legacy configuration from earlier satellite communication standards for analog frequency modulation (FM) transmissions, wherein analog FM transmissions require a higher carrier-to-noise ratio (CNR) (on the order of 14 dB), and are therefore more susceptible to spectral re-growth effects from traveling wave tube amplifier (TWTA) non-linearity and other imperfections. Actually, the designed guard band has provided more than enough margin for analog FM transmissions to reject spread signal energy due to adjacent channel interference.
In comparison, the digital DBS signal requires a lower CNR, on the order of 7.6 dB with the prevailing quadrature phase shift keying (QPSK) modulation and concatenated forward error correction (FEC) codes with a code rate of 6/7. With the advent of turbo codes, which provide higher coding gains than conventional codes, the required CNR drops even lower, to around 5.4 dB for the same code rate. Again, other factors being equal, signals with smaller CNRs are less susceptible to noise and interference effects. Therefore, the existing guard band for the current DIRECTV® frequency plan may be reduced or even eliminated with only a small impact on communication performance. Eliminating the existing guard band would increase spectral efficiency by a factor of 21%. A throughput increase of this ratio would be achieved by increasing the symbol rate without the need to increase the code rate.
Consequently, there is a need in the art for methods and systems that allow for efficient use of a fragmented signal spectrum, such as from a plurality of non-contiguous guard bands. There is also a need in the art for methods and systems that reduce FEC code latency. The present invention satisfies these needs.